Unsaturated fatty acids are important constituents of natural fats. One of their most important representatives is linoleic acid which is a C18 acid with two double bonds. In 1978, researchers at the University of Wisconsin discovered that isomers of this fatty acid where the double bonds are conjugated have particularly beneficial effects on health. These isomeric octadecadienoic acids are collectively referred to in the scientific literature as conjugated linoleic acids (abbreviation: CLA) and have recently attracted increasing attention (NUTRITION, VOL.: 19/NR. 6 1995). They are responsible for a reduction in the fatty content of tissue and support the build up of muscle. In addition, they are said to be anti-carcinogenic and to stimulate the immune system.
Conjugated linoleic acids are present as constituents in various foods. Their principal sources are animal foods, although significant quantities of CLA are also present in milk and dairy products. In addition, CLA have also been found in various oils and fats, their concentration in vegetable oils being significantly lower than in animal fats (J. Food Compos. Anal. 5, 185–197 (1992)).
Since the content in these foods can vary considerably, efforts have been made to add synthetically produced conjugated linoleic acid to foods. EP 0 950 410 A1 discloses a process for isomerizing linoleic acid in the form of an alkyl ester rich in linoleic acid with a strong base, preferably with alkali metal alcoholates, in a water-free medium. The alkyl esters are obtained as described in the book by G. Dieckelmann and H. J. Heinz “The Basics of Industrial Oleochemistry” (ISBN 3-89355-008-9).
The conjugated linoleic acid or the corresponding ester is worked up by neutralization with an acid in aqueous solution. It has been found that, where esters are subjected to the isomerization, a higher yield of conjugated linoleic acid is obtained. Accordingly, the isomerization is generally carried out on the methyl ester using alkali metal methanolates. After the addition of water and acid, phase separation occurs, the conjugated linoleic acid being obtained as the organic phase.
Where working up is carried out in this way, however, the problem generally arises after isomerization that the soaps formed during the reaction prevent clearly defined phase separation when water is added because an emulsion is formed. In addition, the aqueous phase is contaminated by the methanol released which has to be removed at considerable expense for environmental reasons.
Now, the problem addressed by the present invention was to provide a process which would simplify the isolation of conjugated alkyl fatty acid esters after isomerization. Thus, the reaction solution would be prevented from foaming intensively or forming emulsions. In addition, the use of solvents would be limited or avoided altogether; in particular, no water-solvent mixtures would be formed. The purity of the end products would also be increased, i.e. products with a relatively low acid value would be obtained.